1. Field of the Invention
This invention pertains to drilling of boreholes and more particularly to means for controlling apparatus in the borehole and to receiving information sensed by sensors in the borehole.
2. Discussion of the Prior Art
The commerical importance of petroleum exploration and the increasingly inhospitable geology which appears economically interesting have led to many schemes for determining the direction of a hole being drilled and the characteristics of the strata it is traversing. Electrical apparatus in the hole must be supplied with energy and controlled; and observations are preferably transmitted to the surface without the necessity of raising the drill, although many devices propose to record in a bore-hole recorder the readings of instruments which will be available only too late to avoid the mistakes they chronicle. The prior art teaches the use of acoustic impulses through the drill string; conductors of different number in the drill cable; carrier schemes of various sorts to relay information out of the hole; the superposition of variously identifiable pulses on sinusoidal power voltages to transmit intelligence in either direction; different schemes for multiplex or phantom circuit transmission in either direction.
U.S. Pat. No. 3,938,129 of Smither, assigned to the assignee of the present application, discloses the use of a power line to transmit data from the hole to the surface by changing the phase relation of the voltage applied and current supplied at the surface. This is done in the hole by switching a susceptance in parallel with the normal down-hole load to draw an out-of-phase current large enough to be identified at the surface as representing a binary pulse. In particular, it describes two different types of phase detectors which may be used in the present invention.
U.S. Pat. No. 3,959,767 of Smither et al., assigned to the assignee of the present application, teaches the use of phase modulation to transmit binary-coded data back from the hole to the surface, and further teaches the use of the surface power supply to control down-hole apparatus from the surface. This is done by suppressing either a positive or a negative half-cycle of the power voltage. This is detected in the down-hole apparatus.
No other art believed more nearly related to the present invention is known to the applicants.